The present invention relates to screened image reproduction and more particularly to a method and apparatus for electronically generating a screened reproduction of an image.
Electronic screening for image reproduction is well known in the art. According to a well known technique described in U.S. Pat. No. 4,456,924 of the present assignee, for each screened dot, a multiplicity of coordinates of a laser plotter are translated into screen-cell coordinates. A corresponding cell memory is preloaded by threshold values, defining a cell memory matrix. Input digitized scanned density values of an image, such as a color separation, are compared with the threshold values, cell by cell. The results provide an on/off control input for a laser plotter.
U.S. Pat. No. 4,149,183 describes an electronic halftone generator wherein a halftone signal is generated by pulse width modulating or comparing the scanned or video signal with a periodic analog signal having two frequencies and phases to create a dot pattern output which is a function of the frequency and phase of the two combined modulating signals. The halftone reproduction generated has variable dot configurations that are controllable to enable both rotation of the dot pattern and geometric modifications of the dot pattern.
U.S. Pat. No. 5,079,721 of the present applicant and assignee describes apparatus for generating a screened reproduction of an image which has met with significant commercial success.
The present invention relates to an improved method for generating a screened reproduction of an image.
There is provided in accordance with a preferred embodiment of the present invention a method for generating a screened reproduction of an image comprising the steps of:
providing a representation of an original having input density values representing the grey levels of various locations of the original for a given color separation;
defining a desired screen dot arrangement for the image; and
writing screen dots in a line by line fashion, wherein each screen dot is made up of a plurality of line segments whose length and location determines the dot configuration and whose length and location is determined by employing the input density values of the original and the desired screen dot arrangement,
the method being characterized in that at least some of the plurality of line segments are shifted from an otherwise generally straight line configuration so as to cause them to conform to a desired overall dot shape.
Preferably, at least some of the line segments include shifts so as to include a plurality of mutually offset generally straight line portions.
Further, at least some of the line segments include shifts so as to include at least one curved line portion.
In accordance with a preferred embodiment of the present invention, at least some of the line segments include shifts at a location interior of the dot and spaced from edges of the dot.
Preferably, an amount of offset produced by shifting of the line segments is less than an amount that the shifting location is spaced from the edges of the dot.
In accordance with a preferred embodiment of the present invention some of the plurality of line segments include shifts which produce a greater offset than the offset produced by shifting of others of the plurality of line segments.
There is also provided in accordance with a preferred embodiment of the present invention apparatus for generating a screened reproduction of an image comprising:
a grey level representation generator, providing a representation of an original having input density values representing the grey levels of various locations of the original for a given color separation;
a screen dot arranger, defining a desired screen dot arrangement for the image; and
a marker, writing screen dots in a line by line fashion, wherein each screen dot is made up of a plurality of line segments whose length and location determines the dot configuration and whose length and location is determined by employing the input density values of the original and the desired screen dot arrangement, characterized in that:
at least some of the plurality of line segments include shifts from an otherwise generally straight line configuration so as to cause them to conform to a desired overall dot shape.
Preferably, the marker is operative such that at least some of the line segments include shifts so as to include a plurality of mutually offset generally straight line portions.
Even more preferably, at least some of the line segments include shifts so as to include at least one curved line portion.
In accordance with a preferred embodiment of the present invention, the marker is operative such that at least some of the line segments include shifts at a location interior of the dot and spaced from edges of the dot.
Preferably, the marker is operative such that an amount of offset produced by shifting of the line segments is less than an amount that the shifting location is spaced from the edges of the dot.
In accordance with a preferred embodiment of the present invention, the marker is operative such that some of the plurality of line segments include shifts which produce a greater offset than the offset produced by shifting of others of the plurality of line segments.
The apparatus of the present invention may be an imagesetter or digital printing apparatus.